Approximately 36 million Americans report having some degree of hearing impairment. Hearing loss is associated with social isolation, depression, cognitive decline, and economic cost due to reduced work productivity. Understanding ways to optimize communication in listeners with hearing impairment is therefore a critical challenge for speech perception researchers. A hallmark of recent research has been the development of the concept of listening effort, which emphasizes the importance of cognitive processing during speech perception: Listeners with hearing impairment can often understand spoken language, but with increased cognitive effort, taking resources away from other processes such as attention and memory. Unfortunately, the specific cognitive processes that play a role in effortful listening remain poorly understood. The goal of the current research is to provide a more specific account of the neural and cognitive systems involved in effortful listening, and investigate how these factors affect speech comprehension. The studies are designed around a framework of lexical competition, which refers to how listeners select a correct target word from among the possible words they may have heard (Was that word cap or cat?). Lexical competition is influenced by properties of single words (words that sound similar to many others, like cat, are more difficult to process), the acoustic signal (poorer acoustic clarity makes correct identification more difficult), and individual differences in cognitive processing (lower inhibitory ability makes incorrect targets more likely to be perceived). Neuroanatomically, these processes are supported by dissociable regions of temporal and frontal cortex, consistent with a large-scale cortical network that supports speech comprehension. Importantly, individual differences in both hearing impairment and cognitive ability interact with the type of speech being processed to determine the level of success a listener will have in understanding speech. The current research will involve collecting measures of hearing and cognition in all participants to investigate how individual differences in these measures impact speech perception. Converging evidence from behavioral studies, eyetracking, and functional magnetic resonance imaging (fMRI) will be used to explore the cognitive and neural basis of speech perception. Aim 1 evaluates the relationship between lexical competition and listening effort during speech perception. Aim 2 characterizes multiple cognitive processes involved in processing degraded speech. Aim 3 assesses how individual differences in hearing and cognition predict speech perception, relying on a framework of lexical competition to inform theoretical interpretation. These studies will show a relationship between lexical competition and the cognitive processes engaged when processing degraded speech, providing a theoretically-motivated framework to better explain the challenges faced by both normal-hearing and hearing-impaired listeners.